The present invention relates, in general, to a method of processing a compressed audio signal. More particularly, this invention provides a method for skipping and repeating audio frames of an MPEG audio bit stream to synchronize the audio presentation time with the system time.
Presently, audio decoder systems for decoding an MPEG audio bit stream such as that specified, ISO/IEC 13818-1 (Information technologyxe2x80x94Generic coding of moving pictures and associated audio information: Systems reference number ISO/IEC 13818-1:1996(E)) provides a means for conveying information needed to synchronize audio presentation units (audio frames) to a system time. As can be appreciated, the synchronization of audio frames to the system time ensures that compressed audio frames are decoded and presented at the same time as their corresponding video frames. Depending on the particular program content and the particular decoding hardware and/or software that is being used, the corresponding audio and video portions of a television program may not be decoded at the same time and, so, the audio and video portions may fall out of synchronization. This lack of synchronization may be detected as a disparity between the presentation time of an audio frame and the system time.
In order to synchronize digital audio a process of repeating or skipping audio frames is employed. For example, if the audio frame presentation time is lagging behind the system time, it may be necessary to skip audio frames to re-synchronize the audio presentation time with the system time. If the audio frame presentation time is leading the system time, it may be necessary to repeat audio frames to likewise re-synchronize the audio presentation time with the system time.
Presently, audio decoders skip or repeat audio frames as needed. Thus, when the audio presentation time is not synchronized to the system time the next audio frame is skipped or repeated. Depending on the audio signal that is being presented, skipping or repeating an audio frame may be unduly distracting to the listener. If, for example, an audio frame in a loud segment of the program is skipped or repeated, it may create a noticeable audio artifact.
Accordingly, there is a need for an audio processing technique for reducing the audible effects of skipping and repeating audio frames.
The present invention is embodied in a method for synchronizing an audio frame presentation time with a system time in an audio decoder that receives a sequence of audio frames. The method first identifies a time disparity between the system time and the presentation time for one of the audio frames. Responsive to the identified disparity, the method identifies at least one audio frame in the sequence of audio frames having an amplitude value that is less than a threshold amplitude value and processes the identified audio frame to reduce the identified disparity between the audio frame presentation time and the system time.
It is to be understood that both the foregoing general description of the invention and the following detailed description are exemplary, but are not restrictive, of the invention.